1. Field of the Invention
The present invention relates to a small-sized power generating apparatus such as a gas turbine power generating apparatus, and more particularly to a method of operating such a power generating apparatus.
2. Description of the Related Art
Recently, the electric power market has been liberalized under deregulations of electric power. Accordingly, power supplies that can be locally distributed have been attracted considerable attention. Small-sized gas turbine power generating apparatuses have been employed as such power supplies. In the gas turbine power generating apparatus, fuel and compressed air are supplied to a gas turbine engine to rotate the gas turbine engine at an ultrahigh rotational speed of, for example, 100,000 rpm. When the gas turbine power generating apparatus is thus operated, a power generator, which is coupled directly to the gas turbine engine, generates alternating-current (AC) power having a frequency much higher than 50 Hz or 60 Hz of a commercial AC power supply system. Therefore, an output of the power generator is rectified into direct-current (DC) power by a rectification device, then converted into AC power having a frequency, a voltage, and a phase of the commercial AC power supply system by an inverter device, and sent to the commercial AC power supply system.
More specifically, a rotational speed of the power generator is increased at the time of starting of the power generating apparatus. Accordingly, an AC output voltage of the inverter device is also increased. When the AC output voltage of the inverter device becomes equal to a voltage of the commercial AC power supply system, a switch for connecting an output of the inverter device with the commercial AC power supply system is closed so as to send an output of power generation to the commercial AC power supply system.
FIG. 1 is a block diagram showing an interconnection device in a conventional power generating apparatus for connecting an output of the inverter device with the commercial power supply system. The interconnection device has a current detector (current transformer; CT) 231 connected to output terminals of an inverter 208 in the inverter device for detecting an output current of the inverter device, a filter circuit 234 connected to the current detector 231, and a voltage detector (potential transformer; PT) 232 connected to the filter circuit 234 for detecting an output voltage of the inverter device. The filter circuit 234 includes reactors and capacitors. The interconnection device also has an interconnection switch S1 connected to the voltage detector 232, a voltage detector (potential transformer; PT) 233 connected to the interconnection switch S1 for detecting a voltage of the commercial AC power supply system, a switch S2 connected to the voltage detector 233, and terminals 210 for connecting the interconnection device with the commercial AC power supply system.
At the time of starting of the power generating apparatus, the switches S1 and S2 are opened. At the time of interconnection of the power generating apparatus and the commercial AC power supply system, the switch S2 is first closed. Thus, a voltage waveform of the commercial AC power supply system is detected by the voltage detector 233. At that time, the inverter 208 controls its output voltage waveform, which is detected by the voltage detector 232, so as to be equal to the voltage waveform of the commercial AC power supply system. When these voltage waveforms accord with each other, the switch S1 is closed so as to interconnect the power generating apparatus and the commercial AC power supply system via the filter circuit 234, which is disposed near the inverter 208 with respect to the interconnection switch S1.
As shown in FIG. 2, the filter circuit 234 has a closed circuit including reactors L and capacitors C. Thus, an output waveform of the inverter 208 varies in phase and voltage depending upon properties of the filter circuit 234 to thereby produce a slight difference between a voltage waveform of the inverter 208 and a voltage waveform of the commercial power supply system. Accordingly, an inrush current is produced to cause a failure of interconnection.
As described above, in a conventional method of interconnecting the power generating apparatus and the commercial power supply system, an output voltage of the inverter device and a voltage of the commercial power supply system are detected by the voltage detectors 232 and 233, respectively, and the switches S1 and S2 are closed when these voltages accord with each other. However, it is difficult to precisely detect whether the voltages of the inverter device and the commercial power supply system are identical with each other. If the voltages of the inverter device and the commercial power supply system do not accord with each other, then an overcurrent may flow in power switching elements in the inverter device. Therefore, an overcurrent trip may occur and interfere with operation of the power generating apparatus.
Further, in a conventional power generating apparatus, when the gas turbine engine is to be started (or stopped), the power generator may be employed as a motor for the gas turbine engine by using an output of the inverter 208. However, actuation of the power generator is influenced by the filter circuit 234 because the filter circuit 234 cannot be separated from the inverter 208. Therefore, in order to eliminate the influence from the filter circuit 234, it is necessary to provide a mechanism for separating the filter circuit 234 or the capacitors C from the inverter 208.